The diesel sounds nice, but I can't say i'd be sold over the 3.5 eco. I believe the eco sees full boost at 1750rpm the same as the diesel, and I use 87oct in the cooler months and when i'm not towing. I only use 93 when it's hot or I am towing something. So like stated above, I don't know the the slight mpg advantage of the diesel would offset the higher price of diesel enough.
Ultimately, the deciding factor for me would probably be longevity. Am I going to have fewer problems to worry about with the Diesel over the Eco. Not sure if it is still a problem, but previously the Eco was plagued with timing chain issues and condensation buildup issues after driving in the rain causing violent misfires. (I experienced that several times and eventually had to replace a catalytic converter because of it).
If I knew I was going to have fewer to no issues with the diesel, i'd probably go that route, but outside of that, i'd probably stick with a 3.5L Eco.